Ring laser angular rate sensors or ring laser gyroscopes are well known in the art for use in inertial navigation systems. Specifically, the ring laser gyroscope is used to sense annular rotation and thus communicate signals indicative of such rotation to subsequent systems. Ring laser gyroscope operation is further described in U.S. Pat. No. 3,373,650, issued to Killpatrick. Generally, the ring laser gyroscope is comprised of a block which supports two counterpropagating electromagnetic waves, or light beams or laser beams. Rotation of the ring laser gyroscope about an axis normal to the plane containing the counterpropagating light beams causes a phase change in these light beams. The phase change in the light beams is indicative of rotation and detection of this phase change produces a direct measurement of such rotation.
As is well known to those skilled in the art, performance is detrimentally effected by a condition known as lock-in. Lock-in occurs when the counterpropagating light beams approach one another in frequency and phase, causing the two beams to resonate together, thus losing their independence.
U.S. Pat. No. 4,152,071, issued to Podgorski and assigned to the Assignee of the present invention, teaches an apparatus to minimize lock-in by optimally positioning to laser beam path within the laser cavity. As disclosed in U.S. Pat. No. 4,152,071, it is first necessary to establish a pathlength control loop to maintain the laser beams at a maximum DC intensity. This pathlength control loop utilizes one mirror transducer to adjust the dimensions of the laser cavity.
To minimize lock-in and further improve the performance of the gyroscope, a lock-in control loop or Random Drift Improvement loop (RDI loop) is utilized to adjust the lasing path within the gyroscope cavity. Lasing path adjustment is accomplished by simultaneously moving two mirrors within the cavity. (Mirrors are moved using well known transducers such as those taught in U.S. Pat. No. 3,581,227 issued to Podgorski, U.S. Pat. No. 4,915,492 issued to Toth, or U.S. Pat. No. 5,148,076 issued to Albers et al.) For example, as one mirror is moved outward, a second mirror is moved inward. These mirrors are repositioned so as to minimize the Single Beam Signal (SBS). The SBS is the AC component of the laser intensity.
While the apparatus and control method disclosed in U.S. Pat. No. 4,151,071 does provide many improvements to the performance of the gyroscope, it will be recognized that the improvements will not be realized when the gyroscope is subject to high input rates or when dithering is not used. The SBS, which is used to control the RDI loop, is only modulated when the gyroscope nears a zero input rate. If the input rate exceeds the peak dither rate of the gyroscope, the gyroscope does not experience zero input. Furthermore, as the input rate increases, the magnitude of the SBS decreases. When the SBS decreases to a certain level, the RDI control loop cannot close, thus eliminating the purpose of this control.